[Rasch] RSM & PCM

Anthony James luckyantonio2003 at yahoo.com
Wed Mar 10 22:39:26 EST 2010

I was just wondering how PCM accomodates unequal distances when we do 
not model them. We just sum up correct responses on each polytomy and 
analyse it. A sum score is in fact given to the analysis and not 
modelled distances among items. Doesn't here a PCM reduce to an RSM?



--- On Wed, 3/3/10, Anthony James <luckyantonio2003 at yahoo.com> wrote:

From: Anthony James <luckyantonio2003 at yahoo.com>
Subject: [Rasch] RSM & PCM
To: rasch at acer.edu.au
Date: Wednesday, March 3, 2010, 2:17 AM


Dear All, 

I know that
this is a very old and probably a boring question for many of you. But I need
to know this 

What is the
difference between rating  scale model
and partial credit model? 

What I have
gathered is that in RSM the distances between the points on the scale is equal
and this distance is the same for all the items in the instrument. That is, the
ability difference needed to endorse 3 rather than 2 is the same as the ability
difference needed to endorse 5 rather than 4. Right? 

however, the distances between points on the scale is unequal  both within the items and between the items in
the instrument. That is, the ability increment to score 3 on an item rather
than 2 is not the same as the ability increment needed to score 6 rather than
5. And these distances are unequal among  the items in the test. Right?





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